Ionization chamber for high-tension alternating current operation



Dec. 3, 1968 J. CHAMEROY 3,414,726

IONIZATION CHAMBER FOR HIGH-TENSION ALTERNATING CURRENT OPERATION Filed NOV. 5, 1964 2 Sheets-Sheet 1 A.C. SOURCE 0 C. SOURCE TUNED AMPLIFIER I INVENTOR Jinn! CHAMEROY BY W ATTORNEYS Dec. 3, 1968 J. CHAMEROY 3,414,726

IONIZATION CHAMBER FOR HIGH'TENSION ALTERNATING CURRENT OPERATION 2 Sheets-Sheet 2 Filed Nov. 5, 1964 U'IIIIIIIIIA 'IIIIIIII/I/I,

H. m M EE M w c m J ATTORNEYS United States Patent 3,414,726 IONIZATION CHAMBER FOR HIGH-TENSION ALTERNATING CURRENT OPERATION Jean Chameroy, Guyancourt, France, assignor to Socit Anonyme dite: Groupement Atomique Alsacienne Atlantique, Le Plessis-Robinson, France Filed Nov. 3, 1964, Ser. No. 408,567 Claims priority, application France, Nov. 9, 1963, P.V. 953,293 6 Claims. (Cl. 250-8345) The present invention relates to an ionization chamber which operates under high-tension alternating current.

The utilization of alternating current for the high-tension supply of ionization chambers offers undoubted advantages. Unfortunately, if the ionization chamber considered is of the usual type, one important drawback arises which cannot be avoided. In fact, alternating currents pass through residual capacitances of the chamber and interfere with the measurement of the ionization current, especially if this latter is small compared with the stray currents.

In order to eliminate the stray current from the ionization current, it is possible to take advantage of the fact that these two currents which have the same frequency are phase-displaced and, to balance the capacitance of the ionization chamber by means of a variable capacitance in opposition. However, the separation of the two phasedisplaced currents is a delicate operation, particularly if the current to be eliminated is of much higher intensity than the current to be retained. In addition, the capacitance of an ionization chamber is essentially fluctuating and a strict compensation external to the chamber is difficult to carry out. The utilization of an ionization chamber of the ordinary type therefore does not give satisfactory results except with a sufficiently high ionization current.

The purpose of this invention is to provide a special ionization chamber which is capable of overcoming this serious drawback.

To this end, the ionization chamber in accordance with the invention is essentially characterized in that it comprises one collecting electrode and two high-tendon electrodes. Two identical compartments are formed be tween said high-tension electrodes and said collecting electrode, and the two high-tension electrodes are energized with alternating-current potentials which are energized with opposite phase.

In various embodiments of the invention, the electrodes can have a general cylindrical shape or flat shape.

In one embodiment, the collecting electrode is placed between the two high-tension electrodes.

In another embodiment, the high-tension electrodes are located on the same side with respect to the collecting electrode, each high-tension electrode having one-half the useful length of said collecting electrode.

One form of an ionization chamber in accordance with the invention is described hereinafter with reference being made to the accompanying drawings, wherein:

FIG. 1 is a diagrammatic view of the ionization cham ber, as shown in transverse cross-section;

FIG. 2 is a diagram which explains the cycle of operation;

FIG. 3 is a cross-sectional view of an alternative elec* trode arrangement for the ionization chamber; and

FIG. 4 is a cross-sectional view of another alternative electrode arrangement.

In the example of embodiment which is represented in FIG. 1, the ionization chamber is of cylindrical shape. The chamber is essentially constituted by two high-tension electrodes 1 and 2 each forming a portion of cylinder and located on each side of a collecting electrode 3 which 3,414,726 Patented Dec. 3, 1968 ice is also of cylindrical shape. The two electrodes 1 and 2 are identical and located at an equal distance from the collecting electrode 3. There are thus formed between said high-tension electrodes 1 and 2 and said collecting electrode 3 two ionization compartments 4 and 5 which are identical and in opposition to each other.

The electrodes 1 and 2 are connected to a high-tension alternating-current source 10 and are supplied in opposite phase.

On the other hand, the collecting electrode 3 is maintained at a constant voltage and connected to a system for amplifying and recording the ionization current.

The cycle of operation of this chamber is shown dia grammatically in FIG. 2 by means of two graphs wherein the top graph indicates the voltages applied to the different electrodes, while the bottom graph represents the ionization current which is collected on the collecting electrode 3.

The high-tension electrodes 1 and 2 are supplied with alternating current in opposite phase and are brought to alternating current potentials V and V as represented by two opposite sine waves. The mean valve V of these voltages is therefore constant. The potential V of the collecting electrode 3 is also constant but is different from the mean value V In fact, the ionization current in each compartment 4 and 5 is at maximum value when the potential difference V -V or V V within said compartment is at least equal to the saturation voltage V of the ionization chamber. The ionization current of the chamber is therefore at maximum value when the difference between the mean potential V and said constant potential V is equal to said saturation voltage.

In order to obtain this maximum eir'iciency of the ionization chamber with high-tension values which are as low as possible, it is consequently an advantage to ensure that the amplitude of the alternating-current potential applied to the electrodes 1 and 2 is of the order of four times said saturation voltage V,.

In FIG. 2, it has been assumed that these two conditions have been combined and the initial instant t has been considered as that in which the electrodes 1 and 2 are at the same potential which corresponds to V which is distant from V by the saturation voltage V At this moment, maximum ionization currents which are equal and in the same direction are generated within the compartments 4 and 5. The collecting electrode therefore supplies a maximum ionization current I,,=I

Starting from this initial instant t the potential V of the electrode 1 deviates from the voltage V of the collecting electrode 3 while the potential V of the electrode 2 comes closer to said voltage.

At the instant t the potential V of the electrode 2 coincides with that of the collecting electrode V while the electrode 1 is at the potential 2V, with respect to the aforesaid collecting electrode 3. The compartment 5 which separates the electrode 2 from the electrode 3 therefore supplies no ionization current while the compartment 4 supplies the same current as before.

It is this current I /Z which is collected on the collecting electrode.

At the instant t which corresponds to maximum ampli tude, the two high-tension electrodes are brought relatively to -the collecting electrode to voltages which are either equal to or higher than saturation voltage but which are in opposition. The two compartments of the ionization chamber deliver ionization currents which are substantially equal but in opposite direction. The ionization current which is collected on the collecting electrode is substantially zero.

At the instant t in which V and V are equal, the conditions of t, and the current 1 /2 supplied by the single compartment 4 are again met with. At the instant L, in which V and V are equal to V the current is I At the instants t t t and t the curves V and V are exactly reversed with respect to the instants t t t t but the currents which are collected on the electrode 3 are exactly the same as these different instants. The second portion of the curve I therefore reproduces the firsts.

It is therefore plain that the fundamental frequency of the ionization current which is collected is doublethat of the supply frequency and therefore of the frequency of the stray currents. Moreover, the capacitances between the electrodes 1 and 3 on the one hand and 2 and 3 on the other hand are substantially equal and subjected to the same fluctuations. The compensation of stray currents which are due to the capacitances therefore takes place inside the ionization chamber.

As will be apparent, it is possible to collect on the collecting electrode 3 a current which is filtered at the desired frequency which indicates solely the intensity of ionization within the chamber.

The advantages of the ionization chamber as designed in the manner which has just been described, as compared with an ionization chamber of conventional design which is supplied with direct current, are as follows:

(1) In an ionization chamber which is supplied with direct current, it is essential to make provision for a guard ring between the high-tension electrode and the collecting electrode in order to deviate the currents which are otherwise liable to pass from one electrode to the other through the insulators and thus produce a high level of background noise. In the ionization chamber which is described in this invention, this requirement no longer exists. In fact, the leakage currents through the insulators have a continuous current component and an alternating current component having a frequency which is one half that of the ionization currents. Such leakage currents are therefore filtered at the output of the collecting electrode.

(2) In the case of an ionization chamber which is supplied with direct current, and above all in the case of measurement of low currents, it is necessary to have a high input impedance on the amplifier, which makes it necessary to have in the interior of the chamber, and possibly also in the connecting cable, a very high insulation resistance in order that the ionization current should effectively pass through the input impedance.

In the case of the ionization chamber which is described in this specification, this essential requirement is much less stringent since the input impedances in alternating current are much lower than in direct current. This advantage is of particular interest in the case of utilization of an ionization chamber at high temperature inasmuch as the resistivity of the insulators decreases rapidly with the temperature.

(3) At the outlet of an ionization chamber, the cable itself forms an ionization chamber and supplies a current which, in the case of direct-current supply, can prove troublesome and which, in the case of alternating-current supply, is filtered.

(4) A direct-current amplifier is subjected to a drift which makes it necessary to allow said amplifier to heat up for a long time before it can be put to use. This disadvantage does not exist in the case of alternating current amplifiers.

In the example shown in FIG. 1, it has been assumed that the ionization chamber had a cylindrical shape and that the two electrodes 1 and 2 were each on one side of the collecting electrode 3. However, it will be understood that other arrangements of the electrodes could be employed without thereby departing from the scope of the invention.

For example, the electrodes could each be constituted by a cylinder which is placed concentrically with the collecting electrode 3, the electrode 1 being located outside said collecting electrode while the electrode 2' would accordingly be located inside this latter as shown in FIG. 3.

In accordance with another form of embodiment the electrodes could be fiat.

Another arrangement would consist in placing the two high-tension electrodes on a same side of the collecting electrode, each of these electrodes thus corresponding to a portion of said collecting electrode in order to ensure that the two ionization compartments are placed side by side as shown in FIG. 4 or one above the other.

Irrespective of the mode of arrangement adopted, the ionization current will always have a frequency which is double that of the supply current and the compensation will be automaticall effected in the interior of the ionization chamber.

The applications of an ionization chamber as hereinabove described are numerous and several of these applications will now be mentioned:

In the case of an ionization chamber for the measurement of gamma radiation flux, the two high-tension electrodes 1 and 2 are connected (as shown in FIG. 1) to the two outputs of a push-pull generator 10 having a frequency F. The collecting electrode 3 is connected to the central point M of the generator through the intermediary of a stabilized direct-current high tension 12 and the input of the amplifier 14 is tuned to the frequency 2F.

In order to design a chamber for the measurement of neutron flux which is not compensated for gamma radiation flux, the arrangement is the same as that which has been described above. The ionization chamber is filled with boron trifiuoride or else the walls of its two ionization compartments 4 and 5 are lined with boron. In the example which is contemplated in FIGURE 1, the interior of the electrodes 1 and 2 and the exterior of the electrode 3 are accordingly coated each with a layer 6, 7 and 8 respectively.

When said ionization chamber is compensated for gamma radiation flux, use is made of a second collecting electrode at the same potential as the first and forming with the high-tension electrodes 1 and 2 two new ionization compartments solely for gamma radiation. Accordingly, in the example of FIGURE 1, the said collecting electrode would be a cylinder which surrounds the two electrodes 1 and 2. The reception would be carried out on a differential amplifier which measures the difference between the currents derived from the first collecting electrode (neutron flux+gamma radiation flux) and the current which is derived from the second collecting electrode (gamma radiation flux).

An ionization chamber for the safety system of a nuclear reactor is designed on the basis of a non-compensated ionization chamber since the gamma radiation flux is of little importance. An impedance, for example, a capacitance, is interposed between one of the high-tension electrodes and the corresponding output of the push-pull generator. In this manner, compensation for leakage currents is dispensed with and there is collected at the output of the collecting electrode on the one hand a current having a frequency P which is collected on an amplifier which indicates the good state of the ionization chamber and, on the other hand, a current having a frequency 2F which is collected on another amplifier which triggers the reactor safety devices.

As has been stated earlier, the chamber which has been described lends itself to the very numerous applications which usually call for the use of conventional ionization chambers and especially gas circulation chambers, whether of the simple or differential type. This latter application is extremely useful when the circulation of hot gases is concerned, by reason of the advantage which is conferred by the ionization chamber described in that this latter makes it possible to reduce stray currents as a result of compensation of the capacitances within the interior itself of the chamber.

I claim:

1. An ionization chamber adapted to be energized by high voltage alternating current comprising:

two substantially identical high voltage electrodes;

a collecting electrode;

two substantially identical ionization compartments,

each said compartment being defined between a different one of said high voltage electrodes and said collecting electrode;

means for maintaining said collecting electrode at a constant voltage;

means for supplying one of said high voltage electrodes with a first alternating current; and

means for supplying the other of said high voltage electrodes with a second alternating current of the same amplitude and opposite phase as said first alternating current;

whereby an alternating ionization current is generated at said collecting electrode, having a frequency double the frequency of the supply current.

2. An ionization chamber as defined in claim 1 wherein said collecting electrode is of cylindrical form and said high voltage electrodes are curved members arranged to be concentric with the axis of said collecting electrode.

3. An ionization chamber as defined in claim 2 wherein said high voltage electrodes have the shape of a portion of a cylinder and are arranged externally of said collecting electrode.

4. An ionization chamber as defined in claim 2 wherein said high voltage electrodes are of cylindrical shape, one of said high voltage electrodes being located inside said collecting electrode and the other high voltage electrode being located around said collecting electrode.

5. An ionization chamber as defined in claim 1 wherein all said electrodes are flat in shape and arranged in parallel.

6. An ionization chamber as defined in claim 5 wherein said high voltage electrodes are located on the same side of said collecting electrode to form two adjacent substantially identical ionization compartments.

References Cited UNITED STATES PATENTS 2,986,636 5/1961 Carlson et a1 25083.6 X 3,067,350 12/1962 Stebler et a1 250-83.6 X 3,264,154 4/1966 Harrison 25083.6

RALPH G. NILSON, Primary Examiner.

A. B. CROFT, Assistant Examiner. 

1. AN IONIZATION CHAMBER ADAPTED TO BE ENERGIZED BY HIGH VOLTAGE ALTERNATING CURRENT COMPRISING: TWO SUBSTANTIALLY IDENTICAL HIGH VOLTAGE ELECTRODES; A COLLECTING ELECTRODE; TWO SUBTANTIALLY IDENTICAL IONIZATION COMPARTMENTS, EACH SAID COMPARTMENT BEING DEFINED BETWEEN A DIFFERENT ONE OF SAID HIGH VOLTAGE ELECTRODES AND SAID COLLECTING ELECTRODE; MEANS FOR MAINTAINING SAID COLLECTING ELECTRODE AT A CONSTANT VOLTAGE; 